An applanation tonometer is a widely used diagnostic instrument in eye care, serving as a standard component of routine eye examinations. Its primary function involves precisely measuring the intraocular pressure (IOP) within the eye. This measurement provides valuable information about the eye’s internal fluid balance. Understanding this pressure is a fundamental step in assessing overall eye health and identifying potential concerns.
Measuring Eye Pressure
The fundamental principle guiding applanation tonometry is the Imbert-Fick Law, which states that for an ideal, thin-walled, dry sphere, the pressure inside can be accurately determined by the force required to flatten a specific area. The Goldmann applanation tonometer applies this concept by flattening a small, precise area of the cornea. This method is considered a gold standard due to its consistent accuracy.
The tonometer operates by gently bringing a flat-tipped probe into contact with the cornea. It measures the force needed to flatten a standardized area, typically 3.06 millimeters in diameter. This specific diameter is chosen because at this size, the opposing forces of corneal rigidity and capillary attraction (from the tear film) tend to cancel each other out, enhancing the accuracy of the pressure reading.
To visualize the flattened area, yellow fluorescein dye is applied to the eye’s surface, usually with numbing drops. A blue cobalt light illuminates this dye, causing it to glow. The eye care professional observes two green semicircles through the tonometer’s eyepiece. By adjusting a dial on the tonometer, they apply force until the inner edges of these semicircles align, indicating the precise flattening of the target area. The reading on the dial directly corresponds to the intraocular pressure in millimeters of mercury (mmHg).
Clinical Applications
Applanation tonometry is widely used for detecting and monitoring eye conditions, with a focus on glaucoma. Elevated intraocular pressure is a risk factor for glaucoma, a disease that can damage the optic nerve and lead to irreversible vision loss. Regular IOP measurements allow eye care professionals to identify individuals who may be at higher risk and require further diagnostic evaluation.
Measuring eye pressure is also important for tracking treatment effectiveness in patients diagnosed with glaucoma. Consistent monitoring helps determine if current medications or interventions are adequately controlling the pressure and preventing further optic nerve damage. Adjustments to treatment plans are often made based on these readings over time.
Beyond glaucoma, applanation tonometry is used in other clinical situations. It can be used to monitor eye pressure in patients taking certain medications, such as corticosteroids, which are known to sometimes cause an increase in IOP. The test is also performed following eye surgeries, like cataract removal, to ensure the pressure remains within a healthy range during the recovery period. While a high IOP reading signals the need for further investigation, it does not confirm a glaucoma diagnosis, underscoring the importance of a comprehensive eye examination.
Preparing for the Test and What to Expect
Before an applanation tonometry exam, the eye care professional will prepare your eyes. This typically involves applying numbing eye drops to the surface of each eye. These drops quickly desensitize the cornea, preventing pain during the measurement. Fluorescein dye, a harmless yellow substance, is often included with or applied after the numbing drops.
Once your eyes are prepared, you will be asked to sit at a slit lamp. You will rest your chin on a chin rest and your forehead against a headrest to keep your head steady. The eye care professional will then position the applanation tonometer, attached to the slit lamp, in front of one eye.
During the test, the tonometer’s probe gently touches your cornea for a few seconds. You will see a blue light, and the eye care professional will look through an eyepiece, making minor adjustments to the instrument. You may feel a slight pressure or light touch, but numbing drops prevent discomfort. After the measurement, your vision may be temporarily blurry or have a yellow tint from the drops, which resolves within a few minutes.
Understanding Your Eye Pressure Reading
The result of an applanation tonometry test is a numerical reading in millimeters of mercury (mmHg). This number represents the pressure inside your eye. For most individuals, a normal intraocular pressure falls within 10 to 21 mmHg. Readings within this range indicate that the fluid pressure inside the eye is balanced.
A reading outside this range, particularly an elevated one, suggests that the intraocular pressure may be higher than average. An elevated reading does not automatically lead to a glaucoma diagnosis. Instead, it indicates that further evaluation is warranted to determine the cause and assess potential risk to your vision.
Your eye care professional will consider your eye pressure reading in conjunction with other diagnostic tests. These may include an examination of your optic nerve for signs of damage, visual field tests to check peripheral vision, and a measurement of your corneal thickness. A comprehensive assessment incorporating all these findings allows the professional to make an accurate diagnosis and develop an appropriate management plan.